Monday, June 26, 2017

A Beginner's Guide to Solar System Photography with the Celestron 127 SLT (and other Alt/Az Scopes)



If you did any reading at all on forums before buying your telescope, you invariably ran across statements like this:

"An alt/azimuth scope is fine for visual observing, but you can't do astrophotography with them!"

What they mean, generally, is "You can't photograph nebula and galaxies using long exposures." And they are absolutely right. The Alt/Azimuth mount is a terrible choice for long exposures of the sky. You can, however, have an absolute blast taking pictures of some of the planets, and you can take terrific pictures of the moon. These objects are actually quite bright,and there is some very clever software available for free that make it possible to get much better planetary and lunar photos than you might suspect. There are limits, of course, and your results probably won't match what the guys with scopes and mounts costing thousands of dollars will get - but it's a lot of fun, and the results can be pretty respectable if you are careful.

Earlier this year I bought a Celestron 127SLT. I have been having a ton of fun shooting the moon, Jupiter, and Saturn from my front yard. I drove to a darker site on a good night and viewed and photographed a comet. The following documents the process I went through to learn how to get the best results I can from my very modest telescope. I hope others might find it useful. 

This is distilled from many hours of reading the excellent Cloudy Nights forums, and a lot of practice. I am far from an expert on any of this - I have been at it about 5 months now. This article is an attempt to save the reader some time by gathering pointers to the most useful information in one place. I welcome feedback and suggestions for improvement.



Saturn, photographed with my Celestron 127SLT/ZWO ASI290 MC/1.5x Barlow

Overview

Altitude/Azimuth (Alt/Az) mounts are the astronomy name for a pan/tilt mount. They let you rotate the scope around a base, parallel to the horizon, and point it up and down, just like a standard photo tripod does. This is easy to make, and very intuitive to use. The 127SLT (and other scopes in the SLT line) has a motorized Alt/Az Go-To mount, meaning that once you help the mount figure out where it is and which way it is pointed, it can automatically find objects in the sky, and track them for you.

The motion of an Alt/Az mount  does not match what's happening with sky objects, though. An Alt-Az mount will introduce a rotation of your frame. As a result, any single exposure picture of stars you take that is over 15 seconds will start to blur, and become comma-shaped. Stars are rather dim, so long exposures are required to capture them well.

This limitation isn't a problem for planets and the moon, though. They are plenty bright, so you can take short exposures. In fact, as we'll see, the best amateur planetary and lunar images are actually created by digitally combining the best frames from high speed video.

Taking good pictures of Jupiter, Saturn, and the moon are well within the reach of a modest Alt-Az telescope like the SLT line, or the smaller NexStars, Meade also makes comparable lines for introductory to mid-level scopes. I will use the 127SLT for this article, because it's what I have, but the approach and limitations should be similar for most 4-6" scopes of this type.

For capturing the data from the camera, I have used SharpCap 2.9 for this article since I prefer it. You will find similar functionality in the very popular capture software Firecapture.

Power

The SLT series of scopes is reputed to be pretty rough on AA batteries. I read that they would start to slow after a single session of observing, so I chose to skip them entirely. I bought a portable 12V jump starter with an included 5V USB output and the 12V power cable for my telescope. I have not regretted it. I have run it in extremely cold weather and across 3 nights without recharging.

Camera

I did some initial experiments with taking pictures with a Raspberry Pi camera through the telescope, just to learn the basics and see if I liked it. I did, so I chose the ZWO ASI290MC. It has a USB3 port, allowing for higher frame rates on bright targets where you are not limited by the exposure time. As we will see, within limits, the faster you can take frames of the target, the less noise your final picture will contain.

I also added an IR cut filter which is installed on the back side of the camera's mounting tube. This keeps dust off the sensor, and drops IR light. IR light focuses a little differently than visible light, so IR light can cause slight blurring in your images. I installed mine on the inside end of the mounting tube, leaving the other end for attaching other filters or lenses. The IR cut filter is the slightly red glass shown below.




I'd recommend removing the 90-degree mirror diagonal if your scope has one. You don't need it, since you'll be looking at the laptop screen, and it flips the image right to left. You also lose a bit of light - most inexpensive diagonals are about 90% efficient optically. Installed this way, my scope/camera combination produces correctly oriented images.




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